Current work in my lab focuses on the transcriptional regulatory pathways that regulate chondrocyte formation and maturation, elucidation of how mechanical loading regulates gene expression in the joint, identification of target proteins to block progression of osteoarthritis, and the regulation of DNA methylation by Wnt and FGF signals.

Chondrogenesis: Most of the bony tissue in vertebrates is initially molded upon a cartilage template which undergoes a stereotypic maturation process where immature chondrocytes undergo maturation, hypertrophy and apopotosis and are replacement by bone tissue; a process termed endochondral ossification. In contrast articular chondrocytes remain immature and do not normally undergo endochondral ossification. We are studying how the initial cartilage template is induced and are trying to elucidate how chondrocytes “decide” whether to undergo maturation, which leads to endochondral ossification, or remain immature, as in the articular cartilage of our joints. The induction of chondrogenesis in the embryo is regionally controlled by the combination of Shh, Wnt, FGF, and BMP signals. We are studying how these signaling molecules regulate the expression of the prochondogenic transcription factor Sox9 by controlling the epigenetic landscape of this locus, how cell shape and the polymerization state of the actin cytoskeleton controls chondrocyte induction and Sox9 gene expression, and the transcriptional circuits that regulate the process of cartilage maturation, endochondral ossification, and maintenance of articular cartilage. In addition, we are both identifying new transcription factors that control chondrocyte maturation and studying how the activity of these transcription factors are regulated by signaling molecules. Finally, we have recently identified a stem cell population for articular cartilage and are trying to elucidate the parameters that control the proliferation and maintenance of the articular cartilage.

Regulation of gene expression by mechano-signaling: Lubricin, is a secreted proteoglycan encoded by the Prg4 locus, that is abundantly expressed by superficial zone articular chondrocytes and has been noted to be both sensitive to mechanical loading and to protect against the development of osteoarthritis. We have recently demonstrated that Fluid Flow Shear Stress (FFSS) increases secretion of extracellular PGE2, PTHrP and ATP (by epiphyseal chondrocytes) which together engage both PKA and Ca++ regulated signaling pathways, that work in combination to promote CREB-dependent induction of Prg4, specifically in superficial zone articular chondrocytes. Because running and Fluid Flow Shear Stress both boost Prg4 expression in a COX-2-dependent fashion, our results suggest that mechanical motion may induce Prg4 expression in the superficial zone of articular cartilage by engaging the same signaling pathways activated in vitro by Fluid Flow Shear Stress that promote CREB-dependent gene expression in this tissue.

Regulation of DNA methylation by Wnt and FGF signals: The formation of cartilage is restricted to the core of the limb bud mesenchyme by ectodermal Wnts, which can irreversibly silence expression of the prochondrogenic transcription factor Sox9. In contrast, fibroblast growth factor (FGF) signals from the apical ectodermal ridge maintain the competence of chondrogenic precursors to undergo chondrogenesis once these cells go out of range of ectodermal Wnt signals. We have found that Wnt signals induce both a repressive chromatin mark (H3K27me3) and DNA methylation over the Sox9 promoter and that Wnt-induced irreversible silencing of the Sox9 gene requires DNA methylation of this locus, which is specifically countered by FGF signals. We are studying how these two crucial signaling pathways antagonistically regulate DNA methylation of both Sox9 and other loci during development.